The present invention relates to a reverse-bias control circuit for blocking a surge voltage generated by turn-off of a voltage-driven switching element such as a power metal-oxide-semiconductor field-effect transistor (MOSFET) or an insulated gate bipolar transistor (IGBT). Such transistors may be used as high-speed switching elements of inverters for controlling, e.g., machine-tool motors, robots, or air conditioning devices, or in outage-protected power systems for office automation devices and medical equipment.
The present invention represents a major improvement over the prior art which is discussed herein with reference to FIG. 5 (a circuit diagram for a conventional switching circuit with an IGBT as a voltage-driven switching element) and FIG. 6 (a timing diagram for turn-off in a conventional switching circuit).
When a voltage-driven switching element such as the IGBT 1 is to be switched by applying a bias voltage V.sub.GE from the drive circuit 2 between the gate terminal G and the emitter terminal E of the IGBT, it is known that the speed of turn-off can be increased by dropping the turn-off time t.sub.off to 1 microsecond or less, e.g., by changing the bias voltage V.sub.GE step-wise from the forward-bias voltage +V.sub.GE to -V.sub.GE. As a further benefit during turn-off, malfunctioning due to so-called single-transistor noise can be prevented.
Concomitantly, however, the rate of change (di/dt) of the collector current IC during turn-off may become so great that a surge voltage V.sub.SG is generated by superposition, on the wave front of the voltage V.sub.CE between collector C and the emitter E, of a voltage resulting from resonance of the collector-current change with the main-circuit internal inductance of the module, or with the external inductance in the module-connecting wiring. Breakdown failure can occur if the surge voltage exceeds the withstand voltage of the IGBT.
It is known that the di/dt-characteristics of a voltage-driven switching element such as an IGBT, for example, have a reverse-bias voltage dependence, and that di/dt can be reduced by decreasing or stepping down the reverse-bias voltage V.sub.GE. However, simply decreasing the reverse-bias voltage results in an increased turn-off delay time t.sub.d (see FIG. 2) and in increased loss, so that the range of safe reverse-bias operation is narrowed. Inconveniently, moreover, when two IGBT's are connected in series in an inverter circuit, the dead-zone time for preventing short circuits from above and below must be increased, because the device response characteristics are impaired.